THE DIGESTIVE Olid ASS 



16! 



four sharp-pointed canines (ka/nins), aiid back of the 

 canines the eight pre-molars (pre-mo'lers) Bhaped for 



tearing and crushing, while the remainder of the teeth, 

 twelve in number, are the flat-topped molars which do 

 most of the grinding of the food. 



Care of the teeth. — We all know that the teeth are 

 hard. That, however, does not prevent them from becom- 

 ing broken by carelessness or accident, or from decaying 

 because of neglect. When the teeth are not cleaned, a 

 substance called tartar forms on them, which prevents 

 the bacteria from being rubbed off and sometimes pushes 

 the gums away from the 

 teeth. The bacteria 

 cause food particles to 

 ferment and form acids 

 which dissolve the 

 hard outside covering 

 (enamel) and then 

 rapidly the softer parts 

 of the teeth. This re- 

 sults in toothache, a 

 foul breath, and the im- 

 perfect chewing of the food. The teeth should be brushed 

 after each meal to remove particles of food and particu- 

 larly sugar which ferments easily. At least once a year 

 there should be a visit to the dentist who will remove 

 those portions of teeth that are decayed and will lill 

 cavities, thus preventing further decay of the teeth. The 

 value of good teeth cannot be overestimated. 



The esophagus is a nearly straight tube connecting the 

 mouth with the stomach. It passes through the diaphragm 

 (Figure 208), enlarges, and becomes the stomach. As 

 soon as one swallows, control of the food is lost, 

 and further action becomes involuntary. Two sets of 



Figure 183. — Permanent Teeth. 



